148 A TREATISE ON DREDGES AND DREDCI.MI 



with another smaller tube carrying compressed air, which passed 

 into the discharge columns by an annular orifice. The principle 

 upon which the device works is that a mixture of air and water 

 is formed in the discharge column, which is lighter than the water 

 outside, and the difference in pressure is sufficient to create such a 

 velocity that the discharging current will carry with it a certain 

 amount of the material to be excavated. 



The apparatus can be used to depths of 131 to 164 ft. below 

 the water surface, in suitable material, without exceeding an air 

 pressure of 2 atmospheres. The only conditions for its successful 

 use are a depth of water of at least 10 ft. and a height of discharge 

 having for its practical limit one-third of the depth. The same 

 system can be successfully used to transport materials horizontally, 

 the friction in the conduit being much decreased by the air mixed 

 with water. 



In the dredge designed for the Canal at Uleaborg the excavating 

 apparatus consisted of a hydro-pneumatic dredging pipe, which 

 raised the mixture of water and excavated material, and emptied 

 it into a large cylindrical reservoir, which constituted the forcing 

 apparatus. The dredging pipe, the orifice of which rested constantly 

 upon the bottom, formed the axis of a rigid frame which was guided 

 vertically by the sides of a well at the extremity of the boat. Its 

 upper part was connected with a horizontal pipe which entered 

 the reservoir through a flexible elbow. Near the lower orifice of 

 the dredging pipe there was arranged an annular injector, which 

 introduced compressed air into the pipe. This injection of air 

 produced a suction, while at the same time it formed in the pipe 

 a mixture of air, water and material carried along by the water, 

 a mixture whose density was less than that of the water. It is 

 easily conceived that with a given depth of water it was possible, 

 with the coefficients furnished by experiments, to calculate the 

 volume of air necessary to make the external charge upon the orifice 

 greater than the weight of the column of the mixture ascending 

 above the level of the water to a fixed height. 



The principal advantage of this system is that there is no obstruc- 

 tion possible, as the orifice presents a passage that is smaller than 

 the constant section of the pipe, and no parts in motion are in contact 

 with the excavated material. In this way are avoided two of the 

 inconveniences of pumps applied to dredging, and which cause 

 frequent stoppages and necessitate costly repairs. 



